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lo-buck FCD for MAP equip cars (1986's)


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#1 patra_is_here

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Posted 21 March 2007 - 08:00 PM

i went to the electonics store and bought Zener Diode (1watt, 4.3V) and soldered into the ECU pins bridging the signal from the map, and the "sensor ground" and then soldered a bridging wire from "sensor ground" to the main "ground" for the ECU

the best part about using the zener diode is that it allows all the MAP sensor voltages below 4.3V (aka below 13psi) to still function properly. (unlike the resistor type fix that someone came up with)

daily driven at 15psi now, and a couple pulls to 18psi


make sure you use low temp, thin solder for electronics equipments, and a low temp soldering iron. I use a 25watt commercial grade iron, but, some of the cheapos will work too. just make sure you don't feed too much heat into the pins. you should be able to just melt the solder and make the connections. the key is low temp solder and being quick. if you use something too hot, don't blame me.
______________________________

pull out the ECU from the car. take the cover off.

the zener diode should be soldered in as shown. note that the zener is directional, as indicated by the small line that wraps around the diode. the side with the line is known as the cathode side.

so, on the left hand side, solder the zener to the pin, making sure the line (cathode side) on the zener is facing the pin, then have some wire soldered to the other end of the zener and then solder that wire to the pin on the right side as indicated by the picture.

make sure to route and use the proper amount of wire so as not to apply pressure to the various internal bits on the ecu


Edited by patra_is_here, 19 April 2009 - 05:05 PM.






#2 83stariontravis

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Posted 22 March 2007 - 02:09 AM

Just went back and looked at this... It seems wrong to me...  Shouldn't the diode be in series with the MAP sensor.  This seems to be in parallel...  Does anyone see where I am coming from?  Can someone explain this to me?

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#3 patra_is_here

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Posted 22 March 2007 - 10:02 AM

the diode is basically a gate. anything under 4.3v goes into the ECU as it normally would, when ever the voltage goes about 4.3v (aka above 13psi) the EXCESS gets bled of to ground. so the ECU still sees 4.3v

#4 mikec

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Posted 12 January 2008 - 12:02 AM

Zener diode theory:

A "plain" diode is a one-way wire - it does to electricity what a check valve does to air flow.  Now if you push backwards hard enough on the check valve, eventually its guts will bust apart and air will flow through it backwards.  You can do this with a plain diode too - put enough reverse voltage on it and it'll fry.  The max voltage a plain diode can withstand, in reverse, is listed on the diode's box as "PIV" = "Peak Inverse Voltage."  Anything above that will "lightning bolt" its way through the diode, generally frying the diode in the process.  That PIV is a minimum though; thanks to manufacturing tolerances you might get a diode that can withstand more.  I.e., a diode rated at 50PIV (common rating) might withstand 50.0 volts, or 55 volts, etc. before it fails.  It's rather "random."

A Zener diode though has a very low PIV rating - and it's VERY precise.  That's what makes a Zener useful.  Think of it as a surge protector since that's how it's wired into a circuit: in PARALLEL (and backwards) with the power supply voltage (or in parallel with the boost sensor output for the fuel cut application).  When the boost sensor output is below the zener's voltage, the zener does absolutely nothing.  When the voltage tries to go higher though, the zener suddenly "turns on" and acts almost like a short circuit, bleeding off the voltage so it never gets higher than the zener's rating.  That "regulates" the boost sensor output to nothing higher than "allowable boost" if you pick the right zener diode.

If you think about it, that's exactly how those "surge suppressors" that you plug your PC into should operate.  They use a different device from a zener diode though but the principle is similar: do nothing until a target voltage is reached; for higher voltages act like a short circuit to shunt that overload to ground.

Normally a resistor is used inline between the power source and the downstream circuit protected by the zener; this keeps "infinite" current from flowing through the zener.  

mike c.

#5 Komeuppance

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Posted 15 February 2008 - 02:31 PM

Mike C.  are you reccomending using a resistor in series with the diode??

Is the peak voltage for that circuit 5.0v??

-Robert

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#6 mikec

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Posted 15 February 2008 - 11:55 PM

QUOTE (Komeuppance)
Mike C.  are you reccomending using a resistor in series with the diode??

I doubt the pressure sensor can output much current.  When the Zener trips, and acts like a short circuit, a resistor normally provides a voltage drop protecting the Zener.  However, if the voltage source is whimpy, it won't output enough current to fry the Zener - it'll act like the resistor.  To wire in a resistor, the resistor has to be BETWEEN the voltage source (pressure sensor output) and the Zener+ECU input pins.  It must NOT be wired in series with the Zener - that defeats the purpose of the Zener:

correct:

sensor ouput -->resistor --> Zener & ECU
with the other Zener lead grounded.

Patra's pics show this:
sensor output --> Zener & ECU
with the other Zener lead grounded.

And it obviously works since it works on his car.  (and others).  To add the resistor, you would NOT be able to do it inside the ECU case as Patra's pictures show.  The Zener could still be inside the case (wired exactly as Patra shows it) but the harness wire for the pressure sensor output to the ECU would have to be cut and the resistor spliced into the cut.  A 100ohm resistor probably would be a good insurance policy to make sure the Zener never overloads the pressure sensor.

QUOTE
Is the peak voltage for that circuit 5.0v??

Yes; the pressure sensor is fed by the ECU's "sensor power" output pin which is a regulated 5.0volt signal.  This precisely regulated signal prevents varying battery & alternator voltages from screwing up sensor outputs.

mike c.

#7 Jehu

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Posted 26 March 2008 - 11:23 AM

QUOTE (patra_is_here @ Mar 21 2007, 09:00 PM) <{POST_SNAPBACK}>
daily driven at 15psi now, and a couple pulls to 18psi

Patra,

Have you done anything for the fuel supply on your setup to run 15psi?  Is anything needed to be done to compensate?  How high would be safe with the stock fuel system?  A few more HP sounds good if you don't have to worry about blowing anything up! wink.gif

Edited by Daily Interlude, 26 March 2008 - 11:23 AM.

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#8 patra_is_here

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Posted 27 March 2008 - 02:34 PM

my fuel mods would only be stuff like rebuilt the TB with a light port, and new fuel filters, plus, cleaned the injectors by running carb clean through them. the has a little bit of a port and polish

#9 KO

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